During the negative swing the diode "D" is sinking all the current driven through the load, thus droping some 0.7V or so, thus turning off T7, thus the only thing that keeps the negative swing going is the 250µ bootstrap cap.

Yes this is the purpose of the bootstrap cap. I think another feature that is significant for sound quality is that the large currents flow outside of the PCB as the 2N3055 are wired on the heatsink and the diode D sits also on the heatsink. It could be the crossover distortion that appear at power >30 watts are due to a a loss of capactity and deterioration of the bootstrap cap. I wonder how this circuit performs with power BJTs featuring a much higher fT than the 2N3055.

That would be interesting although I doubt the reliability of simulations.
Anyway I am into a project active speakers and hence am interested in reliable amp topologies that are long term stable and require little to none adjustment procedures.

I found in sims that the size of the bootstrap cap is important to distortion; 22uF I found to be optimal when using a Baxandall diode between output and emitter of pnp driver.

Lowest THD is part of the story, I think you are absolutely right about profile of the harmonics. Using good quality sims with good models, I suspect monotonic decreasing, H2>H3> H4 etc is the best profile.

Significantly, this is essentially a quasi-comp output stage, driven from a Rush Cascode, with its phase reversed outputs. But there is still a fair range of current through the RC; with +20dBU output, I saw a range from 0.68mA to 1.05mA, which impacts on Vbe for the RC be junctions. If this range could be greatly reduced, then distortion overall would be lower.

Hum...it is becoming a bit intransparent...could you provide the sims schematic?
I think in terms of the original schematic , a fully complimentary design would not provide advantages. It is a long time ago when I was involved in theory of semiconductors but the fundamentals haven't changed devices being complimentary in every respect are impossible due to the different modes of conduction the electron hole stuff and the different carrier mobilities.

Total Harmonic Distortion: 0.004540% at +20dBU (14.5Vp) into 8R, a respectable figure.

R1, R2 and R3 are all critical; current through the RC is very touchy. The circuit has considerable OLG at the RC outputs; oscillation is a very real issue yet compensation is not used.

I also found, unsurprisingly, that reducing the bootstrap seriously increased distortion at 20Hz. A compromise at around 100uF seems best, but empirical adjustment would be needed as there will be subjective effects on the bass due to the distortion (up around 0.12% with 22uF).

A fully complementary output stage is certainly possible and it would reduce thd considerably, but at the cost of even order distortion, which is musical if not in large quantities. I'd be surprised if it sounded as good.

Thanks.
Why is there no Baxandall clamp at the emitter of Q4? Does it make any sense
to have about 100 Ohms resistor between v out rail and the junction of the base of Q2 and C2?
About oscillation...could it be that the rather slow 2N3055 in the original design prevents oscillation ( I think just the opposite but my amp shows no oscillation under various loads ).
Anyway the THD is impressive that points to absence of switching distortions.
If it were not for possible oscillations this amp promises to be one of the most easy to built... however if it oscillates it will cost a bunch of power BJTs.

While both upper and lower device pairs are complementary feedback pairs (Sziklais), the output from the top is taken from the emitter of slave, whilst at the lower pair it's taken from the collector of the slave.

These circuit conditions are very different, with the upper pair used as a phase inverted, and the bottom with no phase change. The upper output device is used in emitter follower and the bottom in common emitter, so there is a requirement in the bottom pair to compensate the absence of a hefty BE junction between driver and output device, hence the Baxandall diode on one side but not the other.

Quote:

Does it make any sense to have about 100 Ohms resistor between v out rail and the junction of the base of Q2 and C2??

This is the first even remotely interesting circuit that I have seen at DIYA in a long time.
Attached below is the .ASC (self contained models - ready to run). also , I really would like to make this fully complimentary and able to run with 75v rails (next generation simulation). As it is , it works ... does not switch (see below). I had to offset the AC source 1.25v and added 2 -.1R emitter resistors to see whether it switched , but I seem to have gotten the same results ... no switching and .004-.006% thd. FFT is strange, too...